A solar cell, which is also referred to as a photovoltaic cell, is a semiconductor device converting solar energy into electrical energy. In the situation of energy shortage today, the solar cell is with great prospect, because the solar cell is a green product which causes no environmental pollution and the solar energy is a renewable resource. Currently, eighty percent or more of the solar cells are made from crystalline silicon (monocrystalline silicon and polycrystalline silicon). Therefore, the manufacture of a crystalline silicon solar cell with a high efficiency is of great significance in generating electricity in a large scale by using the solar energy.
Currently, the manufacturing process for the crystalline silicon solar cell has been standardized, which mainly includes the following steps S11 to S16.
The step S11 is to perform chemical cleaning on a surface of a silicon sheet and to perform surface texturing (i.e., surface flocking). An uneven structure is formed, by chemical reaction, on the surface of the silicon sheet which is smooth originally to enhance the absorption of light.
The step S12 is to perform diffusion to form a junction. A P-type (or N-type) silicon sheet is placed into a diffusion furnace, N-type (or P-type) impurity atoms come into contact with a surface of the silicon sheet, penetrate and diffuse into the silicon sheet via gaps between silicon atoms, and accordingly a P-N junction is formed. Electrons and holes may no longer return to where they come from after flowing and a current is consequently generated, that is, the silicon sheet has a photovoltaic effect. The diffusion concentration, the depth of the junction and the uniformity of the diffusion affect electrical properties of the solar cell. A total amount of the impurity diffused into the silicon sheet is measured by sheet resistance. The smaller the total amount of the impurity is, the greater the sheet resistance is, and the lower the conversion efficiency is. For a conventional P-type crystalline silicon solar cell, a diffusion process is merely performed on a front surface of the cell to form the junction. For N-type crystalline silicon solar cell, the diffusion process is also performed on a back surface of the cell to form a back surface field. The P-type crystalline silicon includes P-type monocrystalline silicon and P-type polycrystalline silicon. Similarly, the N-type crystalline silicon includes N-type monocrystalline silicon and N-type polycrystalline silicon.
The step S13 is to perform a plasma etching at the periphery of the silicon sheet to remove a conductive layer which is formed at the edge of the silicon sheet during the diffusion process and would cause a short circuit of the P-N junction.
The step S14 is to perform flat-plate type Plasma Enhanced Chemical Vapor Deposition (PECVD), i.e., to deposit an antireflection film. The antireflection film mainly includes silicon nitride film, silicon oxynitride film and/or titanium nitride film. The reflection of light is reduced by using thin film interference principle, an effect of passivation is caused, a short-circuit current and an output power of the cell are increased, and the conversion efficiency is improved.
The step S15 is to print electrodes. For the conventional P-type crystalline silicon solar cell, usually, a front electrode and a back electrode are printed by using silver paste and a back surface field is printed by using aluminum paste, to collect current and conduct electricity. For the N-type crystalline silicon solar cell, usually, the back surface field is formed during the diffusion process.
The step S16 is to perform sintering. An alloy is formed between printed metal electrodes and the silicon sheet at high temperature, that is, good ohmic contacts are formed between contact surfaces. Accordingly, a series resistance of the cell is reduced, and an output voltage and an output current of the cell are raised. Therefore, the forming of the good ohmic contacts has an important influence on the conversion efficiency of the whole cell.
In the practical manufacturing process, it is found that a certain percentage of cell sheets manufactured with the above approach may have a low conversion efficiency. Here, the solar cell sheet with the conversion efficiency lower than 18 percent is regarded as a substandard inefficient sheet or an inefficient sheet. In existing technologies, the inefficient sheet is sifted out through a sorting test, and packaged and stored as a substandard inefficient product. The conversion efficiency of the solar cell is not fully exploited with the above processing approach, and accordingly the economic benefit is reduced.